1. Field of the Invention
The present invention relates to generally to inspection of irregularity or unsteadiness possibly appearing on the surface of objects, specimens and the like, and more specifically to a method and apparatus for detecting surface irregularity of objects, e.g., less conspicuous spots with imperceptible boundary scattered on an object surface, blot zones of differing density often present on the patternless homogenious surface of monolithic object, flaws on a patternless sheet-like specimen, and positional defects in an array of objects arranged at predetermined pitch. Even more particularly, the instant invention is directed to a method and apparatus capable of examining stains or stain areas, minute flaws and crosswise defects of unclear boundary with no remarkable brightness difference, frequently appearing on the surface of various articles.
2. Description of the Prior Art
As is conventional, in the liquid crystal display hereinafter referred to as "LCD", there occurs an occasion that the color filters thereof partially become bright or dark due mainly to the presence of dyeing stains. Likewise, spots of, e.g., differing density or thickness may be created in case of a monochromic patternless woven fabric attached to the ceiling of motor vehicles or a similar nonwoven fabric patch used as a drug carrier. This holds true for the shadow mask of a color cathode ray tube. Namely, electron beam penetration holes formed through the shadow mask may have irregular configuration or may be distributed in an erroneous spacing or pattern. In addition to the above, minute defects or alien matters are often found in a patternless film or an aluminum sheet for use in making beverage cans.
In accordance with the prior art approach, such a surface irregularity of object articles has been detected through the use of an inspection apparatus that includes a line sensor and an image processor associated therewith. The inspection apparatus is designed to find out surface defects by way of first scanning the surface of the object articles to obtain brightness information for each and every pixel and subsequently determining whether the respective brightness information falls within a permissible range.
As a more specific prior art example, the surface examination device shown in FIG. 1 has been used to detect a stain 2 possibly present on a LCD color filter 1. This device comprises a line sensor 10, an image processor 11 and a display 12 wherein the image processor 11 is, in turn, provided with a plurality of brightness information adders 13, 15, a subtracter 14 and an evaluater 17.
In order to detect the stain as a surface defect by means of the examination device noted above, the line sensor 10 takes a picture of the LCD color filter 1 to gain brightness information G1, G2, G3, . . . and Gn for the respective pixel 21 depicted in FIG. 2. The brightness informations may be graphically represented as in FIG. 3(a) in which the ordinate indicates brightness with the abscissa pixel position. The rising ridge 23 sandwiched between a couple of broken lines 24, 25 in the brightness information curve 22 corresponds to the stain on the LCD color filter 1.
In the next step, it is necessary to select a given number of, e.g., seven pixels G1 through G7, the brightness informations of which are summed by use of the first brightness information adder 13 to gain total brightness value a1. Similarly, the brightness informations for the successive pixels G8 through G14 are summed by use of the second brightness information adder 15 to gain total brightness value a2. The subtracter 14 comes to draw total value a2 from total value a1 in an effort to gain differential data N4. The evaluater 17 will then analyze the differential data N4 to see whether it falls within a permissible range. If the data N4 is determined to fall outside the permissible range, it should be recognized that there would exist a pixel or pixels of irregular brightness among the examined pixels.
In the subsequent step, another group of pixels G2 through G8 shifted rightward by one pixel pitch are selected to sum the brightness informations thereof by use of the first adder 13, thus gaining total brightness value b1. In the same way, the brightness informations for the successive pixels G9 through G15 are summed up by use of the second adder 15 to gain total brightness value b2. The subtracter 14 comes to draw total b2 from total value b1 so as to gain differential data N5.
The above processing is repeatedly performed for the entire pixels with a view to obtaining their differential data. The display 12 plays a part in representing the resultant data in a readily understandable form. FIG. 3(b) is a graphical representation of the differential data obtained through the foregoing process. It can be readily seen that the curve 25 of FIG. 3(b) has a ridge and valley portion 26 sandwiched between the broken lines 24, 25, which is more conspicuous than the ridge 23 of the curve 22 shown in FIG. 3 (a). The same processing as set forth above can be executed in the y-axis direction as noted by an arrow 4 in FIG. 1 in order to two-dimensionally highlight the stain or defect.
The prior art surface defect inspection device exemplified in the foregoing is, however, inherently disadvantageous in terms of the following four aspects.
(1) In case of examining surface defects of an object article with lattice-like pattern as shown in FIG. 4, e.g., LCD color filter 1, the intersection point formed by a detection line 31 of the line sensor 10 and a lattice line 32 of the filter 1 may be erroneously recognized as a surface defect, if there exists a misalignment between the LCD color filter 1 and the detection line 31 of the line sensor 10.
(2) Although it is possible to detect the linear stain 33 extending perpendicular to the detection line 31 of the line sensor 10 as depicted in FIG. 5, this is not the case for the horizontally extending linear stain 34 which is parallel to the detection line 31 of the line sensor 10 as shown in FIG. 6. The reason for the horizontally extending liner stain being undetectable is that there would occur no fluctuation between each of the brightness informations of the horizontal stain.
(3) Lastly, it is a frequent occasion that tiny alien matters or diminutive flaws 62 hereinafter referred to collectively as "miniature defects" may be borne by a sheet-like patternless film or an aluminium sheet for the manufacture of beverage cans, as is apparent from FIG. 7. If the miniature defects 62 have a dimension of less than or slightly greater than the pixel size, it becomes hard to detect the miniature defects by way of solely determining whether the brightness informations for the respective pixel fall within or outside the prescribed range. For the "shading" or high frequency noise 71 would preclude convenient detection of the miniature defects 72, as indicated in FIG. 8.